1. Field of the Invention
This invention relates to hard, wear resistant, biocompatible and haemocompatible coatings on components of ventricular assist and total heart implants that are exposed to conditions of wear in the body and that may be exposed to blood components. The coatings reduce the rate of wear of components of the cardiac implant thereby increasing the life of the implant while at the same time presenting a blood compatible surface to blood components thereby reducing the risk of blood clotting.
2. Description of the Related Art
Heart diseases, many of which cannot be cured by conventional surgery or drug therapy, continue to be a leading cause of death. For the seriously ill patient, heart replacement is often one of the few viable options available. While the National Heart, Blood and Lung Institute (NHBLI) estimates that up to 35,000 patients annually could benefit from heart transplants, only about 2,000 are performed each year, mainly due to a lack of availability of donor organs.
In 1988, the NHLBI began funding research and development for permanently implantable, electrically driven, total artificial hearts (TAHs). The pumping mechanism of the TAHs would be implanted into the chest cavity of the patient and the device would be powered by a battery pack and a small transformer, worn by the patient, which transmits energy to the heart with no physical connections through the skin. The NHBLI funded four separate groups: The Cleveland Clinic Foundation & Nimbus, Inc.; Pennsylvania State University & Sarns/3M; The Texas Heart Institute and Abiomed, Inc.; and the University of Utah. Consequently, four competing designs were developed.
The development of TAHs posed several issues. Firstly, it was necessary to duplicate the action of a human heart, ensure long-term reliability and biocompatibility, while producing a device that fits into the chest cavity in terms of both its total volume and the orientation of its connections to natural vessels in the body. Further, the design must fit a variety of patients, even though each patient is unique. In addition, a convenient and highly reliable power source for the heart must be developed.
Artificial hearts deliver approximately 8 liters of blood per minute at 110 mmHg pressure. It is desirable that the heart, in acting as a pump, should be as efficient as possible. A high level of efficiency means longer battery life and a longer period of operation between recharge intervals. Further, a highly efficient pump could also lead to the development of a smaller pump which facilitates implantation into the chest cavity.
Aside from the purely mechanical, wear, and power supply issues, it is also necessary that the design and materials prevent infection and thrombosis. Blood is a non-Newtonian fluid and its properties, such as viscosity, change with oxygen content, kidney function, and even the age of the patient. Further, plasma contains a suspension of fragile red blood cells which may be caught in artificial valves, or other mechanically stressful areas, thereby destroying these cells. It is therefore necessary to develop a pumping action that does not stress blood components material, and to fabricate the pump from materials that are not only biocompatible, but also "blood compatible" in the sense of minimizing damage to blood components and minimizing the formation of blood clots.
Many of the above comments also apply to ventricular assist devices (VADs), one of which is being developed by the Novacor Division of Baxter Health Care Corp. and which will apparently shortly undergo clinical trials. In the use of a VAD, the patient's heart remains in place while the VAD boosts the pumping pressure of the left ventricle of the heart. Consequently, the VAD is an assist device, rather than a replacement. However, the VAD must be blood compatible for the same reasons as the total artificial heart. Aside from the Novacor VAD, VADs have also been manufactured by Abiomed, Inc. and Thoratec, and Thermo Cardio Systems, Inc.
Several patents relating to TAHs and VADs have issued in recent years. These include U.S. Pat. Nos. 4,769,031; 4,750,903; 4,888,011; 4,902,291; 4,981,484; 4,994,078; and 5,066,300. While each of these patents describes an approach to the design of VADs, TAHs, or heart replacement devices like the Jarvik-7 for use outside the patient's body, none of them specifically address the materials to be used for those components which are subjected to mechanical wear, and microfretting wear, while being exposed to sensitive blood components and being subject to the corrosive effects of body fluids. Indeed, there exists a need for a material that is lightweight, biocompatible, and blood and tissue compatible with a hard surface that is resistant to abrasive wear, and microfretting wear, and the corrosive effects of body fluids, for use in heart assist or replacement devices (including EMHs, VADs and TAHs) to prolong the life of mechanical components while at the same time minimizing any deleterious effect on blood components.